Two types of dyeing can be distinguished in the hair sector. The first type of dyeing is semi-permanent or temporary dyeing, also known as direct dyeing, which uses dyes capable of giving the hair's natural coloration a more or less pronounced color change that may be resistant to several shampoo-washes. These dyes are known as direct dyes; they can be used with or without an oxidizing agent. In the presence of an oxidizing agent, the aim is to effect a lightening dyeing. The lightening dyeing is carried out by applying the mixture, prepared at the time of use, of a direct dye and of an oxidizing agent to the hair, and obtains, by lightening the melanin in the hair, an advantageous effect, such as unifying the color in the case of grey hair, or bringing out the color in the case of naturally pigmented hair.
The second type of dyeing is permanent dyeing or oxidation dyeing. This type of dyeing is carried out with so-called “oxidation” dyes comprising oxidation dye precursors and couplers. Oxidation dye precursors, commonly referred to as “oxidation bases,” are compounds that are initially colorless or only weakly colored, and that develop their dyeing power on the hair in the presence of oxidizing agents added at the time of use, leading to the formation of colored compounds and dyes. The formation of these colored compounds and dyes results either from an oxidative condensation of the “oxidation bases” with themselves, or from an oxidative condensation of the “oxidation bases” with coloration modifier compounds, commonly known as “couplers,” which are generally present in the dye compositions used in oxidation dyeing.
To vary the shades obtained with oxidation dyes, or to enrich them with glints, it is known practice to add direct dyes thereto.
Among the cationic direct dyes available in the field of dyeing keratin fibers, especially human keratin fibers, the compounds whose structure is developed in the following text are already known; however, these dyes lead to insufficient colorations, both regarding the homogeneity of the color distributed along the fiber (“unison”), where the coloration is said to be too selective, and regarding the staying power, where the hair resists various attacking factors, including light, bad weather, and shampooing.